Downhole device for data acquisition during hydraulic fracturing operation and method thereof

ABSTRACT

A downhole system including a downhole device for data acquisition during fracing operations is disclosed. It includes a bypass device ( 102 ) operatively coupleable to for instance a frac plug ( 10 ) and is adapted to allow downhole fluid flow through the bypass device ( 102 ) and frac plug ( 10 ) but also permits the closure of that path whilst providing at least one second fluid path ( 120 ) for fluid communication between at least one data acquisition device ( 108 ) provided to detect at least one physical property such as pressure and a region inside the wellbore located uphole of the bypass device ( 102 ).

The present invention relates to devices and processes used in well drilling. More specifically, the invention relates to a downhole device for data acquisition during a hydraulic fracturing operation and a method thereof. Another aspect of the invention relates to a drop device for data acquisition during a hydraulic fracturing operation and a method thereof.

INTRODUCTION

In drilling operations for the production of oil and gas deposits, operators strive to maximize both the rate of flow and the overall quantity of hydrocarbon that can be recovered from the subsurface formation or reservoir to the surface. Therefore, various stimulation techniques have been developed and one of the most commercially successful techniques today is hydraulic fracturing, also referred to as “fracking” or “fracing”.

The hydraulic fracturing process involves targeting a portion of the strata surrounding the wellbore and injecting a specialized fluid into the wellbore at pressures sufficient to initiate and extend a fracture into the formation. The fluid which is injected through the wellbore typically exits through holes which are formed in the cemented well casing using a special tool known as a perforating gun. As a result, a fracture zone, i.e. a zone having multiple fractures, or cracks in the formation, is created through which hydrocarbon fluids such as crude oil or, more commonly, gas can flow into the wellbore and be produced at the surface. These fractures are extended by continued pumping and are either propped open with sand or other propping agents, or the fracture faces are etched by a reactive fluid such as an acid, or both. These techniques allow hydrocarbons contained in the formation to more readily flow from the fractures into the well bore. The artificially created fractures may be complimented by naturally existing fractures, or by fractures caused by previous or simultaneous stimulation operations in the same or nearby formations. The quality of the fracturing operation obviously has a great effect on the overall success or failure of the well production.

In particular, horizontal oil and gas wells allow the extraction of valuable hydrocarbons at a minimum environmental “footprint”, because more wellheads can be grouped together on one surface location requiring fewer rigs and less surface area disturbance, subsequently making it easier and cheaper to produce the wells.

One of the tools commonly used by some operators of hydraulic fracturing equipment includes specially sized “frac-balls” or “drop balls”, which are injected into a well to block the passage of a previously installed frac plug and therefore close off portions of a well to allow pressure to build up and cause fracturing in a target section of the well above the location of the well blocked by the frac-ball. The frac-plug (or bridge plug) is a downhole tool that is located and set to isolate the lower part of the wellbore. Frac-plugs are usually retrievable so that the lower wellbore section can be temporarily isolated from a treatment conducted on the upper section of the frac-plug (i.e. during fracing operations). Frac-balls may be made of various materials, including G-10 (or other related phenolics), Torlon, PEEK, and other high-temperature thermosets or thermoplastics.

Typically, the material selected is based upon the operators' experience and the chemistry and temperatures within the well. Frac-ball sizes are selected specifically to fit within the well-bore or sliding sleeves of the downhole tools which vary in diameter as the well sections progress from upper to lower sections.

For example, one popular method for creating multiple fractures in a wellbore is the use of frac ports & sliding sleeves. Openhole packers isolate different sections of the horizontal well. A sliding sleeve is placed between each packer pair and is opened by injecting a properly sized frac-ball inside the borehole. Typically, a completion string is placed inside the well. The string includes frac ports and open hole packers spaced to specifications. The spacing between packers may be up to several hundred feet. The packers are actuated by mechanical, hydraulic or chemical mechanisms. In order to activate each sleeve, a properly sized frac-ball is pumped along with a fracturing fluid inside the well. Each ball is smaller than the opening of all of the previous sleeves, but larger than the sleeve it is intended to open. Seating of the frac-ball exerts pressure at the end of the sliding sleeve assembly, causing it to slide and open the frac ports. Once the port is opened, the fluid is diverted into the openhole space outside of the completion assembly, causing the formation to fracture.

At the completion of each fracturing stage, the next larger frac-ball is injected into the well, which opens the next sleeve, and so on, until all of the sleeves are opened and multiple fractures are created in the well. The main advantage of this completion technique is the speed of operation, because it allows activating multiple fractures with a single completion string.

In some alternative completions, the sleeves are sometimes cemented into the hole with acid soluble cement, such that the sleeves are opened by pumping down acid and the fracing operation can thereafter be performed. Also, occasionally, in some other wells, the sleeves can be run with out packers if the formation is of the type that it can be counted on to provide isolation between the sleeves.

In any event, in all completions, the quality of the fracturing operation obviously has a great effect on the overall success or failure of the well production. Therefore, knowing current formation and/or wellbore properties can be an important tool to optimize productivity of the hydrocarbon output and improve the quality and safety of fracing operations. In particular, knowing what was/is happening downhole at each of the perforations during the perforation and fracing process, may be invaluable information in order to improve the quality and safety of the fracing process.

Accordingly, it is an object of the present invention to provide a downhole device and method for data acquisition before, during and after fracing operations. It is a further object of the present invention to recover the downhole device and acquired data from the wellbore.

SUMMARY OF THE INVENTION

Preferred embodiments of the invention seek to overcome one or more of the above disadvantages of the prior art.

According to a first aspect of the present invention, there is provided a downhole device for data acquisition during fracing operations, comprising:

-   -   a bypass device operatively coupleable to a wellbore selective         obturation member and adapted to provide a selectively closable         first fluid path for fluid communication through said wellbore         selective obturation member between a region inside the         throughbore of the wellbore located uphole of said downhole         device and a region inside the wellbore located downhole of said         downhole device when in situ, and     -   at least one data acquisition device, adapted to detect at least         one physical property and operatively coupled to said bypass         device, wherein said bypass device is further adapted to provide         at least one second fluid path for fluid communication between         said at least one data acquisition device and said region inside         the wellbore located uphole of said bypass device.

This provides the advantage that physical properties of the formation and wellbore environment can be measured and recorded before, during and after the fracing operation to provide important downhole information for evaluation. For example, the initial pressure increase after a perforation operation has been completed may provide important information about the structural integrity of the wellbore/casing and opened fractures.

Furthermore, frac pressure and temperature may be monitored during the fracing operation and/or pressure and temperature between different zones may be monitored during all subsequent fracing operations in the wellbore.

Preferably, the bypass device comprises a bypass sub device and the wellbore selective obturation member is typically a plug or packer suitable for use during a fracing operation.

Typically, the second fluid path communication allows fluid communication from an uphole region to a data acquisition device without compromising the frac plug operation for circulating fluid along the first fluid path between an uphole and downhole region in the wellbore until a drop member that is typically selectively introduced into the fluid flow at surface by an operator shuts the first flow path in order to initialize perforation. Preferably, the drop member is a drop ball, dart or the like. Alternatively, the first fluid flow path may be shut by means of a suitably located valve operable from the surface.

The compatible bypass sub device provides the advantage that it can be retro-fitted to any standard frac plug, minimizing cost and maximize ease of use.

The data acquisition device may be operatively coupled to said bypass sub device via a data acquisition device carrier housing mountable to said bypass sub device and may be adapted to accommodate said at least one data acquisition device. This provides the advantage that a plurality of data acquisition devices can simply be placed inside a data acquisition device carrier and be replaced according to current specific requirements.

Furthermore, since the data acquisition device carrier is operatively coupled to the bypass sub such as to provide a direct fluid communication with an uphole wellbore region, no alterations are necessary to the data acquisition devices in order to expose the data acquisition devices to the fluid communication and allow unperturbed measurements of the predetermined uphole wellbore environment. Advantageously, the bottom hole assembly may further comprise a centralising member operatively coupled to said data acquisition device or data acquisition device carrier housing and adapted to centralize and/or rotationally stabilize said data acquisition device within the wellbore. More advantageously, the centralizer member may comprise an anti-rotation element adapted to engage with a compatible uphole end of a second bottom hole assembly located immediately below the first bottom hole assembly in the wellbore so as to prevent rotational movement of said bottom hole assembly with respect to said second bottom hole assembly.

In addition, the centralizer member provides the advantage of centralizing the data acquisition device carrier housing and/or data acquisition device within the wellbore during recovery back to surface and whilst guiding the preceding data acquisition device carrier housing and/or data acquisition device over the top of the next lower bottom hole assembly within the wellbore. The anti-rotation element provides the additional advantage of anchoring the data acquisition device carrier housing and/or data acquisition device such as to prevent spinning during a milling process when recovering the data acquisition devices.

A seat member located within or at an end of the first fluid path may be adapted to engage with a drop member so as to at least unidirectionally selectively prevent fluid flow through said first fluid path. Advantageously, the second fluid communication path may be adapted to bypass said first fluid path.

This provides the advantage that both, the frac plug and the data acquisition device(s) can perform their function without compromising the function of the other. In particular, because the second fluid communication path to the at least one data acquisition device bypasses the first fluid path through the bore hole assembly, the first fluid path can be closed or opened as required during a fracing operation without blocking the fluid communication path to the at least one data acquisition device.

The at least one data acquisition device may comprise an integrated data storage device adapted to record a predetermined maximum amount of data. Advantageously, the data acquisition device may comprise a data interface adapted to permit download at least part of said predetermined amount of data to a remote data storage means. More advantageously, the data acquisition device may be adapted to transmit data to a remote location uphole of said bottom hole assembly within the wellbore.

This provides the advantage that the measured data can be recovered for evaluation either after recovering the at least one data acquisition device to the surface or while the bottom hole assembly is still downhole after or during fracing operations.

The at least one data acquisition device may be any one or all of a pressure data acquisition device and/or a temperature data acquisition device and/or a geophone and/or a micro seismic sensor.

The data acquisition device carrier may be adapted to telescopingly and slidingly irretractably engage with a data acquisition device catcher mechanism when moving along a longitudinal axis inside a pick-up device of the data acquisition device catcher mechanism. Advantageously, the data acquisition device carrier housing may comprise a recess and/or shoulder or other formation provided on or in the outer surface of the data acquisition device carrier housing or may alternatively and preferably comprise a groove located circumferentially around an outer surface of said data acquisition device carrier housing and adapted to cooperate with a spring loaded catcher ring mechanism located inside a pick-up device of the data acquisition device catcher mechanism.

This provides the advantage that the data acquisition device carrier housing and data acquisition device can be recovered using a milling or drilling device or the like (hereinafter simply referred to as “milling device”) comprising a data acquisition device catcher device operable to drill over the bottom hole assembly removing any parts that exceed the inner diameter of the pick-up device but keeping the data acquisition device carrier housing and data acquisition device intact. Furthermore, the catcher ring mechanism ensures that the data acquisition device carrier housing does not fall out of the data acquisition device catcher mechanism during recovery of more than one data acquisition device carrier housing.

The bypass sub device may be located downhole of the frac plug when in situ. Preferably, the data acquisition device and/or data acquisition device carrier housing may be located downhole of said bypass sub device. Alternatively, the bypass sub device may be located uphole of the frac plug and the data acquisition device and/or data acquisition device carrier housing may be operatively located inside a mandrel of the frac plug.

The second fluid communication path may comprise an incompressible fluid sealingly encased within said second fluid communication path and said data acquisition device carrier housing by a membrane located at the uphole end of said second fluid path. The membrane is typically pressure sensitive such that it can transmit the pressure from one side to the other. This provides the advantage that the data acquisition device is safely enclosed in a protected environment, but is still in fluid communication with the uphole wellbore region. The membrane may be sufficiently flexible and diathermic to prevent distortion of the physical properties that are measured with the at least one data acquisition device.

According to a second aspect of the present invention, there is provided a method for acquiring data during fracing operations in a subterranean formation having a wellbore penetrating the formation, the method comprising the steps of:

-   (a) running in a downhole device having a data acquisition device     into a wellbore along with a wellbore selective obturation member; -   (b) placing said downhole device and wellbore selective obturation     member at a location inside the wellbore; -   (c) initiating at least one data acquisition device of said downhole     device for monitoring at least one physical property; -   (d) initiating a fracing operation while monitoring said at least     one physical property, and -   (e) recovering said monitored physical property after the fracing     operation is completed.

Preferably, step (a) of the method comprises running in a downhole device according to the first aspect of the present invention.

Typically, step (b) comprises placing said downhole device and wellbore selective obturation member at a predetermined location inside the wellbore.

Preferably, there is an additional step in between steps c) and d) of:—

-   -   closing a first fluid path for fluid communication through the         wellbore selective obturation member between a region inside the         wellbore located uphole of said downhole device and a region         inside the wellbore located downhole of said downhole device         when in situ;     -   and where step (d) includes monitoring said at least one         physical property through said at least one second fluid path.

This provides the advantage that important data can be determined before, after and during fracing operations and provided to an operator for evaluation to optimize production and/or improve quality and safety of fracing operations, therefore improving output and minimizing cost. Furthermore, recovering the data acquisition device carrier housing and/or data acquisition device instead of destroying or damaging it, provides the advantage that expensive data acquisition devices can be reused thus minimizing costs.

Advantageously, the at least one data acquisition device may be adapted to record a predetermined amount of data of said at least one physical parameter. More advantageously, the data acquisition device may be adapted to transmit said predetermined amount of data to a remote data storage means. Even more advantageously, the at least one monitored physical property may be recovered via a remotely located communication sub device operatively connectable to said at least one data acquisition device.

According to a third aspect of the present invention, there is provided a method for recovering at least one downhole device having a data acquisition device from a downhole location inside a wellbore typically after completing fracing operations, the method comprising the steps of:

-   (a) running a plug milling device, comprising at least one data     acquisition device retrieval mechanism, downhole inside a wellbore; -   (b) engaging a first of said at least one downhole device so as to     irretrievably move a data acquisition device carrier and/or data     acquisition device of said at least one downhole device into and     along a longitudinal axis of said data acquisition device retrieval     mechanism, milling away any parts of said at least one downhole     device exceeding an inner diameter of said data acquisition device     retrieval mechanism, and -   (c) removing said plug mill including said at least one downhole     device from said wellbore.

Preferably, the method of the third aspect of the present invention comprises recovering at least one downhole device according to the first aspect of the present invention.

Preferably, the method according to the third aspect may also include an additional step between steps (b) and (c) of engaging any subsequent second of said at least one downhole device so as to telescopingly and slidingly irretrievably move a data acquisition device carrier housing and/or data acquisition device of said second of said at least one downhole device into and along a longitudinal axis of said data acquisition device retrieval barrel, milling away any parts of said second of said at least one downhole device exceeding an inner diameter of said data acquisition device retrieval barrel.

This provides the advantage that at least one data acquisition device and/or data acquisition device carrier and be reusably recovered from its downhole location. Preferably, a plurality of data acquisition device carrier housings and/or data acquisition devices are recovered on a single downhole “trip”. Furthermore, the recovering method of the third aspect of the present invention provides the advantage for embodiments thereof that the data acquisition device retrieval barrel protects the at least one data acquisition device carrier housing and/or data acquisition device while pulling them out of the wellbore.

The milling device and data acquisition device retrieval barrel may be adapted to be operated on coiled tubing or drill pipe or any other suitable work string.

The data acquisition device retrieval barrel may comprise at least one catcher means adapted to engage with said data acquisition device and/or data acquisition device carrier housing of said bottom hole assembly so as to prevent downhole movement of said data acquisition device and/or data acquisition device carrier housing with respect to said data acquisition device retrieval barrel in situ. Advantageously, the catcher mechanism may be at least one spring biased catcher member adapted to engage with at least one groove, recess of shoulder or the like located circumferentially around or on the outer surface of said data acquisition device and/or data acquisition device carrier housing.

According to a fourth aspect of the present invention, there is provided a drop device for data acquisition during fracing operations, comprising:

-   -   an obturation member adapted to actuatingly and/or sealingly         engage with at least one downhole tool, and     -   at least one data acquisition device operatively coupled to said         obturation member and adapted to detect at least one physical         property.

Advantageously, the obturation member may be adapted to sealingly engage with a seat member of said downhole tool so as to prevent fluid communication through said downhole tool between a region inside the throughbore of the wellbore located uphole of said obturation member and a region inside the wellbore located downhole of said obturation member when in situ. Preferably, the data acquisition device may be located uphole of said obturation member when in situ.

This provides the advantage that physical properties of the formation and wellbore environment can be measured and recorded before, during and after the fracing operation to provide important downhole information for evaluation. For example, the initial pressure increase after a perforation operation has been completed may provide important information about the structural integrity of the wellbore/casing and opened fractures.

Furthermore, frac pressure and temperature may be monitored during the fracing operation and/or pressure and temperature between different zones may be monitored during all subsequent fracing operations in the wellbore. Also, a drop device provides the advantage of improved ease of use and minimized cost, since the drop device is simply pumped down to the required location where the obturation member engages with the determined downhole tool automatically locating the data acquisition tool in the required position within the throughbore.

The data acquisition device may be operatively coupled to said obturation member via a data acquisition device carrier housing that may be removably mountable to said obturation member and adapted to accommodate said at least one data acquisition device.

This provides the advantage that a plurality of data acquisition devices can simply be placed inside a data acquisition device carrier and be replaced according to current specific requirements. Furthermore, since the data acquisition device carrier is operatively coupled to the obturation member such as to provide a direct fluid communication with an uphole wellbore region, no alterations are necessary to the data acquisition devices in order to expose the data acquisition devices to the fluid communication and allow unperturbed measurements of the predetermined uphole wellbore environment.

The drop device may further comprise a centralizing member operatively coupled to said data acquisition device and adapted to centralize and/or fixate and/or rotationally stabilize said data acquisition device within the wellbore. Advantageously, the centralizing member may be operatively coupled to said data acquisition device via said data acquisition device carrier housing. Even more advantageously, the centralizer member may comprise a plurality of spreading elements adapted to spread radially towards and in engagement with the inner wall of the wellbore.

This provides the advantage that the data acquisition tool is securely fixed within the wellbore region that is measured during fracing operation, minimizing the risk that the data acquisition tool is dislodged or tilted, twisted or wedged within the throughbore when exposed to the substantial hydraulic pressures during fracing operation. A data acquisition tool tilted or wedged within the throughbore would also make it more difficult or even impossible to recover. In addition, a data acquisition tool that is not optimally placed within the throughbore may provide biased or falsified measurements.

The drop device may further comprise a flange member operatively coupled to said data acquisition device and adapted to provide a contact surface for fluid flowing through the wellbore. Advantageously, the contact surface may comprise an uphole contact surface adapted to operatively engage with fluid flowing in an uphole direction, and a downhole surface adapted to operatively engage with fluid flowing in a downhole direction when in situ. Advantageously, the flange member may be adapted to actuate said centralizing member and preferably may be adapted to slidingly actuate said centralizing member.

This provides the advantage that the hydraulic energy provided by fluid flowing uphole or downhole can be used to move or remove the drop device to or from a desired location within the wellbore and/or actuate the centralizing member. For example, the hydraulic energy of the fluid flowing downhole may be used to slidingly move the flange member along a longitudinal axis and with respect to the data acquisition tool and/or data acquisition tool carrier housing, compressing and subsequently spreading the plurality of spreading elements into engagement with the interior wall of the throughbore, thus centrally fixating the data acquisition tool within the throughbore. During recovery of the drop device, the hydraulic energy of the fluid flowing uphole may be used to slidingly move the flange member back along the longitudinal axis and with respect to the data acquisition tool and/or data acquisition tool carrier housing expanding and disengaging the plurality of spreading elements from the interior wall of the throughbore so that the drop device can be pushed uphole through the throughbore of the wellbore.

The at least one data acquisition device may comprise an integrated data storage device adapted to record a predetermined maximum amount of data. Advantageously, the data acquisition device may comprise a data interface adapted to permit download of at least part of said predetermined amount of data to a remote data storage means. Even more advantageously, the data acquisition device may be further adapted to transmit data to a remote location uphole of said downhole tool within the wellbore.

This provides the advantage that the measured data can be recovered for evaluation either after recovering the at least one data acquisition device to the surface or while the bottom hole assembly is still downhole after or during fracing operations.

Preferably, the at least one data acquisition device may be any one or all of a pressure data acquisition device and/or a temperature data acquisition device and/or a geophone and/or a micro seismic sensor. Even more preferably, the obturation member may be a drop ball, dart or the like.

According to a fifth aspect of the present invention, there is provided a method for acquiring data during fracing operations in a subterranean formation having a wellbore penetrating the formation, the method comprising the steps of:

(a) providing a drop device according to the fourth aspect of the present invention at a predetermined location and in engagement with a predetermined downhole tool; (b) initiating at least one data acquisition device for monitoring at least one physical property; (c) closing a first fluid path for fluid communication through the downhole tool between a region inside the wellbore located uphole of said drop device and a region inside the wellbore located downhole of said drop device when in situ; (d) initiating a fracing operation while monitoring said at least one physical property through at least one data acquisition device of said drop device, and (e) recovering said monitored physical property after the fracing operation is completed.

This provides the advantage that important data can be determined before, after and during fracing operations and provided to an operator for evaluation to optimize production and/or improve quality and safety of fracing operations, therefore improving output and minimizing cost. Furthermore, being able to fully recovering the data acquisition device instead of destroying or damaging it, provides the advantage that expensive data acquisition devices can be reused thus minimizing costs.

The drop device may be recovered by fluid flowing uphole through the wellbore and/or drill string and engaging with a contact surface of a flange member of said drop device.

This provides the advantage that there is no need for additional equipment and/or energy to recover the drop device and data acquisition tool thus minimizing cost.

Alternatively, the drop device may be recovered by running a milling device, comprising at least one data acquisition device retrieval barrel, downhole inside a wellbore. In particular, during recovery of the drop device, the milling device engages said drop device so as to, for example, telescopingly and slidingly irretrievably move a data acquisition device carrier housing and/or data acquisition device of said drop device into and along a longitudinal axis of said data acquisition device retrieval barrel, milling away any parts of said drop device and/or downhole tool exceeding an inner diameter of said data acquisition device retrieval barrel, and subsequently removes said milling device including said data acquisition device from said wellbore. However, it is understood by the skilled person in the art that any other suitable retrieval barrel and retrieval mechanism may be used to recover and store the drop device and/or at least one data acquisition device when using a milling device.

Alternatively, the drop device may be recovered by securing it with an alternative retrieval device such as a fishing tool or the like on the end of an elongate member such as a wireline or slick line and pulling it out of the hole.

This provides the advantage that at least one data acquisition device and/or data acquisition device carrier can be reusably recovered from its downhole location. Preferably, a plurality of data acquisition device carrier housings and/or data acquisition devices are recovered on a single downhole “trip”. Furthermore, the recovering method of the fifth aspect of the present invention provides the advantage for embodiments thereof that the data acquisition device retrieval barrel protects the at least one data acquisition device carrier housing and/or data acquisition device while pulling them out of the wellbore.

According to a sixth aspect the present invention provides a frac plug comprising a downhole device according to the first aspect of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will now be described, by way of example only and not in any limitative sense, with reference to the accompanying drawings, in which:

FIG. 1 shows a perspective view of a first but less preferred embodiment of a downhole device according to the present invention as including a bypass sub, a data acquisition device carrier and data acquisition device (inside the data acquisition device carrier and therefore not seen in FIG. 1) and a centralizer;

FIG. 2 shows a cross sectional side view of the downhole device of FIG. 1, but with an alternative centralizer;

FIG. 3 shows a perspective sectional view of the bypass sub of FIG. 1 having a first flow path selectively closed by a drop ball;

FIG. 4 shows a sectional side view of the bypass sub of FIG. 3 indicating a (fully open) flow path of the fluid from an uphole wellbore region to a downhole wellbore region;

FIG. 5 shows a sectional side view of the bypass sub of FIG. 3 indicating a (restricted) pressure bypass path of the fluid from an uphole wellbore region to a data acquisition device and/or data acquisition device carrier when the first flow path is selectively closed by a drop ball;

FIG. 6 shows (a) a close up sectional side view of the data acquisition device carrier of FIG. 1 and a data acquisition device (Micro Automatic Gauge) placed inside the data acquisition device carrier, (b) a close up sectional view of the interface port of the data acquisition device, and (c) a close up view of the pressure transducer;

FIG. 7 shows a sectional side view of a second and more preferred embodiment of a downhole device according to the present invention, with the bypass sub mounted to the uphole end of a frac plug and two data acquisition devices placed in series one above the other inside the mandrel of the frac plug;

FIG. 8 shows a sectional side view of the downhole device of FIG. 7 (but with the outer most parts of the frac plug and the data acquisition device not shown), where the bypass sub is mounted to the uphole end of an inner mandrel of a frac plug, where (a) shows the fluid flow from an uphole wellbore region through to a downhole wellbore region when the first flow path is open, (b) shows the fluid pressure path along a second fluid communication path, bypassing the first flow path, from an uphole wellbore region into a data acquisition device carrier (data acquisition device not shown), and (c) shows a front view and section lines of (a) and (b);

FIG. 9 shows (a) a top view, (b) a bottom view, (c) a sectional side view along C-C, and (d) a sectional side view along A-A of the bypass sub of FIG. 8;

FIG. 10 shows a simplified schematic side view of the step of pumping the downhole device of the first aspect of the present invention downhole inside the wellbore in accordance with the second aspect of the present invention;

FIG. 11 shows a simplified schematic side view of the step of closing the first flow path of the downhole device of the present invention by dropping a drop ball into place;

FIG. 12 shows a simplified schematic side view of the step of recovering the downhole device in accordance with the first aspect of the present invention from the wellbore using a milling device (e.g. plug mill) and data acquisition device retrieval mechanism in accordance with a third aspect of the present invention;

FIG. 13 (a) shows a sectional side view of a first (less preferred) embodiment of a milling device and data acquisition device retrieval mechanism of FIG. 12 including spring loaded catching slips and centralising pads;

FIG. 13( b) shows a perspective view; 13(c) a side view and 13(d) a bottom view of the spring based catcher ring of the less preferred embodiment of milling device and retrieval mechanism of FIG. 13( a)

FIG. 14 shows a sectional side view of the pick-up shoe of the data acquisition device retrieval barrel of the less preferred milling device shown in FIG. 13( a);

FIG. 15( a) shows a cross-sectional side view of a second and most preferred embodiment of a milling device and data acquisition device retrieval mechanism in accordance with the third aspect of the present invention;

FIG. 15( b) shows a close up more detailed view of the milling device and part of the retrieval mechanism of FIG. 15( a), particularly showing springloaded catching slips and centralising pads;

FIG. 16 shows a sectional side view of the less preferred milling device and data acquisition device retrieval barrel of FIG. 13( a) at the entry stage when milling over a downhole device of FIG. 1 during recovery of the data acquisition device carrier and data acquisition device, in accordance with the third aspect of the present invention;

FIG. 17 shows a sectional side view of the less preferred milling device and data acquisition device retrieval barrel of FIG. 16, but at the end stage when milling over the downhole device during recovery of the data acquisition device carrier and data acquisition device;

FIG. 18 shows the less preferred milling device and data acquisition device retrieval barrel of FIG. 17, but filled with a recovered downhole device;

FIG. 19 shows the less preferred milling device and data acquisition device retrieval barrel of FIG. 18, but filled with a plurality of recovered bottom hole assemblies that are interlocked via the centralizer of the downhole device;

FIG. 20( a) shows a cross-sectional side view of a third and most preferred embodiment of a downhole device according to the present invention, with the bypass sub mounted within and just below the uphole end of a frac plug and one data acquisition device located inside the mandrel of the frac plug;

FIG. 20( b) shows a different cross-sectional side view of the downhole device of FIG. 20( a) but on a different cross-section plane and FIG. 20( b) therefore shows the second fluid communication pressure path;

FIG. 20( c) shows a cross-sectional side view of the most preferred embodiment of a milling device and data acquisition device retrieval mechanism as shown in FIGS. 15( a) and 15(b), approaching the upper end of the most preferred downhole device of FIGS. 20( a) and 20(b);

FIG. 20( d) shows the milling device and retrieval mechanism of FIG. 20( c) as having milled away the uppermost outermost section of the frac plug of FIGS. 20( a) and (b) and also shows the upper end and also the drop ball of the downhole device of FIGS. 20( a) and (b) as having entered the retrieval mechanism;

FIG. 20( e) shows the milling device and retrieval mechanism of FIG. 20( d) as having further milled away most of the outermost part of the frac plug of FIGS. 20( a) and (b) and also shows the majority of the downhole device as having entered the retrieval mechanism; FIG. 20( f) shows the milling device and retrieval barrel of FIG. 20( e) but now with the downhole device of FIG. 20( e) as having fully entered the retrieval mechanism and also showing the milling device starting to mill the upper and outermost section of a second and lower frac plug such that the uppermost end of the downhole device of the second frac plug is now starting to enter the retrieval barrel;

FIG. 21( a) shows a perspective view of a drop device in accordance with a fourth aspect of the present invention, the drop device including a obturation member (drop ball), a data acquisition tool (logging tool) that is mounted to the drop ball, and a centralizing member that is actuatable by a flange member;

FIG. 21( b) shows a cross sectional side view of the drop device of FIG. 21( a) when engaged with a downhole tool and fixated to the throughbore;

FIG. 22 shows a cross sectional side view of the drop device of FIG. 21 during recovery when disengaged and pushed up by fluid flowing uphole, and

FIG. 23 shows a simplified schematic side view of an alternative recovery method of the drop device in FIG. 21( a) from the wellbore using a milling device and data acquisition device retrieval barrel similar to the one shown in FIG. 12 used for the third aspect of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The following definitions will be followed in the specification. As used herein, the term “wellbore” refers to a wellbore or borehole being provided or drilled in a manner known to those skilled in the art. The wellbore may be ‘open hole’ or ‘cased’, being lined with a tubular string. Reference to up or down will be made for purposes of description with the terms “above”, “up”, “upward”, “upper”, or “upstream” meaning away from the bottom of the wellbore along the longitudinal axis of a work string and “below”, “down”, “downward”, “lower”, or “downstream” meaning toward the bottom of the wellbore along the longitudinal axis of the work string. Similarly ‘work string’ refers to any tubular arrangement for conveying fluids and/or tools from a surface into a wellbore. In the present invention, a coiled tubing string or a drill pipe string is the preferred work string.

Referring to FIGS. 1 to 6, a downhole device 100 is shown comprising a bypass sub tool 102, a data acquisition device carrier 104 and a centralizer 106. The downhole device 100 is less preferred when compared to the more preferred downhole device 200 of FIG. 7 and particularly the most preferred downhole device 250 for reasons to be detailed subsequently. The upper end of the less preferred bypass sub 102 of FIG. 1 is adapted to be mounted to the lower end of any standard frac plug 10 and/or on any manufacturers frac plug. The data acquisition device carrier 104 of FIG. 1 is operatively coupleable to the in use lower or downhole end of the bypass sub 102 and is adapted to provide a housing for at least one data acquisition device 108. The centralizer 106 is mounted to the lower or downhole end 107 of the downhole device 100 and comprises, for example, a centralizer disc 110 and an anti rotation stab 112.

Alternatively, and as shown in FIG. 2, the centralizer 106 may comprise a flange-type member 114 having a depression 116 at the downhole end that is adapted to cooperatively engage with a matching counter part (not shown) coupled to the uphole end 109 of another downhole device 100. The centralizer 106 and matching counter part (not shown) engage such as to guide the downhole end 107 of an upper downhole device 100 into the uphole end 109 of a lower downhole device 100 during recovery of a plurality of bottom hole assemblies 100 placed within a wellbore as will be subsequently described in more detail.

As shown in FIG. 2, the bypass sub 102 comprises a first fluid path 118 which in use is in fluid communication with the fluid flow through path of the frac plug 10 such as to provide a fluid path between an uphole region of the wellbore (that is, the wellbore located above the plug 10) and a downhole region of the wellbore located below the plug 10. The first fluid path is selectively blockable by a drop ball 124 landing on a drop ball seat 121 (see FIG. 3). The bypass sub 102 comprises a second fluid pressure path 120 bypassing the dropball 124 such as to provide fluid communication (particularly to transmit pressure) between the uphole region of the wellbore and the interior cavity 122 of the data acquisition device carrier 104 even in the event the first fluid path 118 is blocked by the drop ball 124, thus allowing measurements of physical parameters of the uphole wellbore environment located above the frac plug 10 at all times during the fracing operation. FIG. 3 shows a detailed perspective section view of the bypass sub 102, the first fluid path 118, the bypassing second fluid pressure path 120 and drop ball 124. It is understood that the bypass sub 102 may comprise more than one first fluid path 118. Similarly, the bypass sub 102 may comprise more than one fluid pressure path bypass path 120. The upper end of the bypass sub 102 of the less preferred downhole device 100 is coupled to the lower end of the frac plug 10 via a fluid tight screwthread 125 having a relatively large diameter and which is preferably larger than the inner cutting diameter “d” of a milling tool 300 or 350 to be detailed subsequently. The upper end of the data acquisition device carrier 104 is coupled to the lower end of the bypass sub 102 via a fluid tight screw thread 126 having a relatively smaller diameter than thread 125 and preferably a smaller diameter than the inner cutting diameter “d” of the milling tool 300 or 350 for reasons which will be discussed subsequently. However, it is understood by the skilled person in the art that any other suitable connection may be used to mount the bypass sub 102 to the data acquisition device carrier 104 and to the plug 10. The data acquisition device carrier 104 and/or the plug 10 may be removably or permanently fixed to the bypass sub 102.

Referring to FIG. 4, the first fluid path 118 through the bypass sub is shown in greater detail when there is no drop ball 124 blocking the first fluid path 118. FIG. 5, shows the second fluid pressure path 120 through into the interior cavity 122 of the data acquisition device carrier 104 when the drop ball 124 blocks the first fluid path 118; as can be seen in FIG. 5, no fluid can flow along or through the second fluid pressure path 120 because the lower end of the cavity 122 is blocked.

FIG. 6 shows a particular and preferred example of a data acquisition device in the form of a Micro Automatic Gauge 108 placed inside the data acquisition device carrier 104. The interface port 128 of the Micro Automatic Gauge 108 is located at the downhole end of the data acquisition device carrier 104 and the pressure transducer 130 is located towards the uphole end of the data acquisition device carrier 104. This particular Micro Automatic Gauge 108 requires no input to acquire data since the sensor of the Micro Automatic Gauge 108 is in monitor mode checking for pressure and temperature changes after a pre-determined period has lapsed for instance in between a second and some minutes such as every 2 minutes. Once a temperature or pressure threshold have been exceeded, the Micro Automatic Gauge 108 automatically switches into record mode sending the acquired data to a memory at a pre-set sample rate. At the end of the acquisition, the Micro Automatic Gauge 108 automatically switches off and goes back into its monitor mode. The Micro Automatic Gauge 108 is available from the applicant, Omega Well Monitoring Limited of Aberdeen, United Kingdom.

FIGS. 7 to 9 show an alternative downhole device 200, where the bypass sub 202 is located vertically above the frac plug 20 such that the in use lower (downhole) end of the bypass sub 202 is mounted to the in use uphole end of the frac plug 20 such that the at least one data acquisition device carrier 204 and/or data acquisition device 208 is located inside the mandrel 22 of the frac plug 20. The downhole device 200 of FIG. 7 is not as preferred as the downhole device 250 of FIG. 20( a) though because the downhole device 200 may impede the setting of the frac plug 20 (which is set with a special setting tool (not shown)) that connects at the top of each frac plug 20 and which is not a problem experienced with the most preferred downhole device 250. In the example shown in FIG. 7, two data acquisition devices 208 are placed inside a data acquisition device carrier 204 that is in fluid communication with a second fluid pressure path 220 (see FIGS. 8 and 9) from an uphole region of the wellbore above the plug 20 to the data acquisition devices 208 (not shown in FIG. 8). A first fluid path 218 runs through the bypass sub 202 and the frac plug 20 between an uphole region of the wellbore located above the plug 20 and a downhole region of the wellbore located below the plug 20 allowing fluid communication between the uphole and downhole region of the wellbore in the absence of a drop ball 224 blocking the first fluid path 218. If a suitably sized drop ball 224 is dropped into the fluid flow at surface, it will come to rest against the upper most end 221 or seat 221 of the bypass sub 202, in which case the first fluid path 218 is blocked, but the data acquisition devices 208 can still experience the parameters of the fluid (such as pressure and/or temperature) via the bypass path 220.

Note that FIG. 8 omits to show a data acquisition device carrier located below the bypass sub 202.

FIGS. 10 to 12 show the placement, operation and recovery of the downhole device 100 step-by-step. In particular, as shown in FIG. 10, the downhole device 100 is mounted to the downhole end of a frac plug 10 (although the downhole device 200 of FIG. 7 would be mounted to the uphole end of a frac plug 20 and the downhole device 250 or FIG. 20( a) would be mounted within frac plug 20) and placed downhole into the wellbore in a region that is determined for perforation and a fracing operation by running them into the wellbore 101 by conventional methods such as lowering on wireline (not shown) or a tubing string such as coiled tubing (not shown) or pumped into the wellbore 101, wherein a plurality of downhole devices 100 may be placed in the required respective zone in a plurality of dedicated regions. The plug(s) 10 (or 20) are then set in the conventional manner to seal against the inner surface of the wellbore 101. Perforation guns (not shown) associated with each downhole device 100 are then detonated in the conventional manner to create fractures within the formation. The pressure and temperature changes caused by the detonation may initiate the data acquisition of the data acquisition device 108, which also starts to record the data. Alternatively, the data acquisition devices 108 may be operated from the surface remotely, or may be on a timer to operate after a period of time has elapsed.

In the event that the perforation is unsuccessful, a second perforation gun is pumped downhole to the first downhole device 100, before blocking the first fluid path with a drop ball 124.

After a successful perforation, as shown in FIG. 11, a first drop ball 124 is pumped downhole such as to engage with the seat 221 provided in the bypass sub 102 or 202 or 252 of a first downhole device 100 or 200 or 250 and block the first fluid path 118 or 218 or 268. Subsequently, fracing fluid including proppants is then pumped downhole and into the fractures of the formation at extremely high pressure, while determining, for example, wellbore pressure and temperature using the data acquisition devices 108 or 208 or 258 by means of being in fluid communication via the second fluid pressure path 120 or 220 or 270. After completing the fracing operation of the first downhole device 100 or 200 or 250, a communication sub tool (not shown) may be moved downhole to connect to the data acquisition device 108 or 208 or 258 and recover the recorded data via a near field transmission means such as RFID or other suitable means. Alternatively, the data may be recovered after physically retrieving the data acquisition device 108 or 208 or 258 to the surface (and this method and apparatus therefore will be described subsequently), or the data may be transmitted uphole via a signal path established, for example, through a wireline or a transceiver (not shown).

A second smaller diameter “d” rop ball 124 or 224, which is adapted to engage with the seat 121 or 221 or 271 of a second, subsequently run in downhole device 100 or 200 or 250, is then pumped downhole to block the first fluid path 118 or 218 or 268 of the second upper most downhole device 100 or 200 or 250 before repeating the fracing operation for that upper most downhole device 100 or 200 or 250 while recording, for example, well pressure and temperature for evaluation. These steps are repeated with further bottom hole assemblies 100 or 200 or 250 until all dedicated formation regions are perforated and fraced.

Once the fracing operations have been completed, as shown in FIG. 12, a tool to mill away the bottom hole assemblies 100, 200 or 250 for example a milling device (i.e. a mill plug) 300, is sent downhole, for example, on coiled tubing or a drill pipe string to recover the data acquisition devices 108, 208 or 258 and pick-up each downhole device 100 or 200 or 250 in turn starting with the uppermost one closest to the surface.

FIG. 13( a) shows a first but less preferred embodiment of a part of a coring-style bit mill 300 in accordance with the present invention as comprising TSP cutters 302, a data acquisition device catcher lower shoe 303 and its data acquisition device catcher 304, an outer barrel 306 and an inner barrel 308. The data acquisition device catcher 304 is, for example, a spring biased catcher ring 304 mounted within the shoe 303 as shown in FIGS. 13( b), 13(c) and 13(d). Alternatively, the two piece shoe 303 and inner barrel 308 can be replaced by a one piece pick-up shoe 312 as shown in FIG. 14 in which case the catcher ring 304 is mounted within a cavity 310 of the pick-up shoe 312. In either case, the catcher ring 304 is adapted to engage with a groove 132 located circumferentially around the data acquisition device carrier 104 or data acquisition device 108, as shown in FIGS. 16 and 17, once the ring 304 has traveled down the outside of the data acquisition device carrier 104 to prevent the data acquisition device carrier 104 from otherwise falling out.

FIGS. 15( a) and 15(b) show the most preferred embodiment of mill 350 and will be detailed subsequently.

FIG. 16 shows the mill 300 of FIG. 13( a) during operation at the stage when the downhole device 100 is just entering the inner barrel 308 or pick-up shoe 312. The mill 300 has milled away and thus removed the outer part of the frac plug 10, 20 and the bypass sub 102, 202 including the relatively large diameter threaded connection 125 (but has not milled the relatively small diameter threaded connection 126). FIG. 17 shows an advanced stage after the data acquisition device carrier 104 has entered the pick-up shoe 312 and the data acquisition device catcher 304 has engaged with the groove 132 of the data acquisition device carrier 104 preventing any unwanted downhole movement of the data acquisition device carrier 104 with respect to the mill 300 than the point shown in FIG. 17.

FIG. 18 shows substantially the whole length of the mill 300 including its upper end and its inner bearing assembly 305 and its inner barrel 308 into which is recovered the data acquisition device carrier 104 and data acquisition device 108 of a downhole device 100 (not shown in FIG. 18). Sections of the frac plug 10, bypass sub 102 and centralizer 106 will be milled away and thus removed such that the downhole device 100 fits into the inner barrel 308 or pick-up shoe 312. The data acquisition device catcher lower shoe 303 may be modified to allow the additional option of taking samples of the wellbore reservoir for analysis.

FIG. 19 shows a mill 300 after retrieval of a plurality of bottom hole assemblies 100. During retrieval each anti rotation stab 112 of a preceding (upper) downhole device 100 engages with a complementary section at the uphole end of the frac plug 10, 20 of a subsequent (immediately lower) downhole device 100 to guide the mill 300 over the next downhole device 100 and prevent rotation of the downhole device 100 with the drill bit 302 of the mill 300 and with respect to the wellbore 101, thus allowing to mill away the outer section of the downhole device 100 that does not fit inside the inner barrel 308 or pick-up shoe 312.

The mill 300 with bottom hole assemblies 100 located therein is then pulled out of the wellbore 101 to the surface, where the data acquisition device carriers can be removed and recovered from the inner barrel 308/pick-up shoe 312 and the data stored therein can be downloaded or otherwise retrieved and analysed.

FIGS. 20( a) and 20(b) show an alternative and indeed most preferred embodiment of a downhole device 250 in accordance with the present invention, where the bypass sub 252 and the data acquisition device 258 are mounted within the throughbore of the frac plug 20 (rather than being mounted below the frac plug 10 as shown in FIG. 2 or respectively above and within the frac plug 20 as shown in FIG. 7 for that embodiment of the downhole device 200). The downhole device 250 is most preferred because it is wholly located within the frac plug 20 and therefore will not impede the setting of the frac plug 20 which typically is set with a special setting tool (not shown) that connects at the top of each frac plug 20.

The bypass sub 252 is coupled via a screwthread 275 provided on its outer surface to a similarly shaped screwthread surface formed on the inner throughbore 21 of the frac plug 20, where the screwthread 275 is a relatively large diameter and which is preferably larger than the inner cutting diameter “d” of a milling tool 350 to be detailed subsequently. The lower end of the bypass sub 252 is coupled to the upper end of the data acquisition device carrier 254 via a fluid tight screwthread 276 having a relatively smaller diameter than thread 275 and preferably a smaller diameter than the inner cutting diameter “d” of the milling tool 350, for reasons which will be discussed subsequently. FIG. 20( a) shows frac ball or drop ball 224 as being seated on ball seat 271 but were the frac ball 224 not present, the first fluid path 268 through the bypass sub 252 can be seen in FIG. 20( a). FIG. 20( b) shows the second fluid pressure path 270 as being present when the frac ball 224 is seated on the ball seat 271 where the second fluid pressure path 270 leads from the throughbore portion of the frac plug 20 located above the frac ball 224 and into the interior cavity 272 of the data acquisition device carrier 254. As can be seen in FIG. 20( b), no fluid can flow along or through the second fluid pressure path 270 because the lower end of the interior cavity 272 is blocked. A membrane (not shown) is typically provided across the second fluid pressure path 270 at some point, where the cavity 272 is filled with clean hydraulic fluid and the membrane acts to seal that cavity 272 but also acts to communicate the pressure of the downhole fluid to the sealed hydraulic fluid.

FIGS. 20( a) and 20(b) also show the data acquisition device such as sensors or gauges 258 as being housed within the interior cavity 272. The data acquisition device 258 is similar in form to the data acquisition device 108 as described above.

The downhole device 250 has a first shoulder 280 formed at its very upper end around the outer circumference of its very upper end such that the shoulder 280 projects outwardly, and a second outwardly projecting shoulder 281 approximately one quarter of the way down the length of the tunnel device 250 and just below the thread 276, and a third outwardly projecting shoulder 282 approximately three quarters of the length down the downhole device 250 and a fourth outwardly projecting shoulder 283 provided at its lower most end where the functions of the first 280, second 281, third 282 and fourth 283 shoulders will be discussed in detail subsequently.

The upper end of the downhole device 250 is secured by means of the screwthread 275 to the inner bore 21 of the plug 20 as described above. The lower end of the downhole device 250 is provided with a downwardly projecting insert 285 and which projects downwardly through an aperture formed in an annular ring centraliser 287 where the centraliser 287 is secured via a left hand thread to the inner bore 21 of the frac plug 20, the left hand thread ensuring that the centraliser 287 can not be rotated out of secure connection with the inner bore 21 if the rest of the downhole device 250 were to rotate or to be caused to rotate in the right hand direction within the inner bore 21. The insert 285 is however not in threaded coupling with the inner bore of the centraliser 287 and therefore it would be possible for the downhole device 250 to rotate within the centraliser 287 if it were caused to do so and the centraliser 287 prevents the insert 285 and therefore the downhole device 250 from moving downwardly through the centraliser 287 and therefore the centraliser axially locks the downhole device 250 in a downwards direction. However, the centraliser 287 does not prevent the insert 285 from moving upwardly through it if for instance the downhole device 250 were pulled upwards with respect to the inner bore 21 of the frac plug 20 and that possibility will be described subsequently. Consequently, the centraliser 287 centralises the lower end of the downhole device 250 within the throughbore 21 of the frac plug 20 and in doing so prevents the insert 285 and therefore the downhole device 250 from moving in a radial direction with respect to the longitudinal access of the frac plug 20 and further prevents the downhole device 250 from moving downwards through the centraliser 287 but does not prevent the downhole device 250 from being moved upwards out of the frac plug 20 were that to be possible for instance were the screwthread 275 to be milled away. As can be seen in FIGS. 20( a) and 20(b) the centraliser 287 has some apertures 289 formed around its annular ring such that when the drop ball 224 is not present and fluid can flow through the first fluid path 268, the fluid can exit the throughbore at the bottom of the frac plug 20 through the apertures 289.

Once the fracing operation with frac plug 20 and downhole device 250 have been completed, as shown in FIG. 20( c), the milling device 350 of FIGS. 15( a) and 15(b) is run into the wellbore for instance on coiled tubing or a drillpipe string to recover the data acquisition device 258 from each downhole device 250 in turn starting with the uppermost one closest to the surface of the wellbore.

The most preferred coring style bit mill or milling device 350 is shown in FIGS. 15( a) and 15(b) as comprising PDC cutting elements 352 (but these could be TSP cutting elements or Tungsten Carbide cutting elements etc,), a data acquisition device catcher lower shoe 353 and its data acquisition device catcher 354 and an outer barrel 356 (the most preferred coring style bit mill 350 not requiring an inner barrel like the other embodiments). As can be most clearly seen in FIG. 15( b) the data acquisition device catcher 354 is a plurality of individual spring loaded catching slips 354 which are normally biased to project into the bore of the bit mill 350 by respective individual springs 357 such that the slips 354 provide an upwardly facing catching surface 359 which as will be described in more detail subsequently, will cooperate with the shoulders 280, 281, 282 and 283 of the downhole device 250 to prevent the downhole device 250 from falling out of the retrieval barrel 361 once the respective shoulder 280, 281, 282, 283 has cleared the slips 354 and therefore the slips 354 secure the data acquisition carrier 254 inside the retrieval barrel 361.

Both the shoe 353 and the outer barrel 356 contain longitudinally extending centralising pads 363 which will act to prevent lateral movement of the data acquisition device carrier 354 as it enters the retrieval barrel 361. As shown in FIG. 15( a) the mill 350 is provided with a crossover sub 351 at its uppermost end to connect the mill 350 to the lower end of a bottom hole assembly on which the mill 350 is run into the wellbore.

FIG. 20( c) shows the mill 350 of FIG. 15( a) during operation at the stage where the mill 350 is approaching the upper end of the frac plug 20 and therefore the upper end of the downhole device 250 and, as can be seen in FIG. 20( c), the mill 350 will shortly start to mill away the upper end of the frac plug 20. The inner diameter (d) of the mill cutting head 350 is chosen such that it is smaller in diameter than the diameter of the relatively large diameter threaded connection 275 but is larger in diameter than the relatively small diameter threaded connection 276 and therefore the mill 350 will mill away the threaded connection 275 such that the rest of the upper end of the bypass sub 252 with a diameter smaller than diameter “d” starts to enter the retrieval barrel 361. The mill 350 is now at the position shown in FIG. 20( d) as having milled away part of the outside of the bypass sub 252 and particularly in FIG. 20( d) the bypass sub 252 has entered the retrieval barrel 361 sufficiently such that the shoulder 280 has just moved past the slips 354 such that the slips 354 have been moved into the groove that is immediately below the shoulder 280 under the action of the springs 357 such that the bypass sub 252 and therefore the downhole device 250 cannot now exit the retrieval barrel 361. Continued downward movement of the mill 350 will continue to mill away the rest of the frac plug 20 such that the second shoulder 281 will move past the slips 354 and further continued downward movement of the mill 350 will result in the slips 354 moving past the third shoulder 282 and subsequently the fourth shoulder 283. By the time slips 354 have moved past the third shoulder 282, the plug 20 is no longer attached to the wellbore via its slips 22U as they have been milled away and that is the position shown in FIG. 20( d). Furthermore, by the time the shoulder 283 passes the slips 354, the lower frac plug slips 22L will have been milled way as well. FIG. 20( f) shows that as having incurred with a first downhole device 250U and also shows that a second lower downhole device 250L has started to enter the retrieval barrel 361 of the mill 350.

Importantly, once the first shoulder 280 of a downhole device 250 has passed the slips 354, the relatively large diameter threads 275 will have been milled away and therefore it would be possible for the mill 350 to pick up the downhole device 250 at any point thereafter because the slips 354 will prevent the first shoulder 280 from passing back through the slips 354 but the downhole device 250 would only be pulled out of the well in that configuration as a last resort because the rest of the downhole device 250 would be protruding out of the lower end of the mill 350 and therefore would be more susceptible to damage as it was pulled out of the wellbore and therefore it is much preferred that the mill 350 does mill away all of the frac plug 20 so that the downhole device 250 fully enters the retrievable barrel 361 and is retained therein by its fourth shoulder 283. The third 282 and fourth 283 shoulders are therefore beneficial particularly when there are at least two downhole devices 250 to recover from the wellbore.

Referring now to FIGS. 21 to 23, an alternative to the downhole devices 100 or 200 or 250 is shown in the form of a drop device 400. The drop device 400 comprises an obturation member, such as a drop ball 402, and a data acquisition device 404 removably and operatively coupled to the drop ball 402. Preferably, a data acquisition device 404 is coupled to the drop ball 402 via a data acquisition device carrier housing 403 that is mounted to the drop ball 402. The data acquisition device carrier housing 403 is adapted to accommodate at least one data acquisition device 404, such as a logging tool. The at least one logging tool may be suitable to determine any one or both of temperature or pressure. Alternatively or additionally, the logging tool may include a geophone and/or micro seismic sensor. Preferably, the data acquisition device 404 and/or data acquisition device carrier housing 403 is located uphole of the drop ball 402 when in use so that the data acquisition device 404 is located within the isolated fracing region. However, a second data acquisition device (not shown) may be secured to (but be located downhole of) the drop ball 402 when in use.

A centralizing member 406 is operatively mounted to the data acquisition device 404 or data acquisition device carrier housing 403. The centralizing member 406 comprises a plurality of spreading elements 408 adapted to engage with an interior wall of the throughbore of the wellbore so as to fixate and centralize the drop device 400 within the throughbore. In particular, the centralizing member 406 is actuated by slidingly compressing the spreading elements 408 until the spreading elements 408 bend towards and engage with the interior wall of the throughbore. The centralizing member 406 is actuated by a flange member 410 that is operatively coupled to the data acquisition device 404 or data acquisition device carrier housing 403. The flange member 410 is adapted to slide along a longitudinal axis of the drop device 400, but is prevented from sliding off the data acquisition device 404 or data acquisition device carrier housing 403. The flange member 410 provides an uphole contact surface 412 and a downhole contact surface 414, both of which are adapted to engage with fluid flowing uphole or downhole, respectively.

It is understood by the skilled person in the art that any other suitable obturation member may be used. In particular, alternatively to the drop ball, the obturation member may be a dart or plug or other device suitable for obturating a packer or shifting a sleeve.

Referring now to FIGS. 21( a) and (b) and 22, during operation, the drop device 400 (having a suitably sized drop ball 402) is injected into the wellbore and pumped down to a predetermined downhole tool 500. The drop ball 402 is sized so as to only sealingly engage with the predetermined downhole tool 500. For example, the drop ball 402 may seal off fluid communication when engaging with a suitably sized seat 502 of the predetermined downhole tool 500. The drop device 400 may actuate other downhole tools (not shown) while on the way.

For example, in a multi stage fracturing system, the drop ball 402 may actuate a hydraulically-activated flow port 504 that is run in conjunction with a fracture port sleeve (not shown), thus allowing specific zones of the wellbore to be isolated and selectively fractured. In particular, the multiple hydraulically-activated flow ports can be successively activated by locating the ball seat 502 with the largest diameter at the top (most uphole) of the line. Starting with the smallest diameter “d” rop ball 402, the flow ports and fracture port sleeves can be isolated and activated one by one.

As shown in FIG. 21( b), the drop ball 402 of the drop device 400 has activated the flow port 504 and is sealingly engaged with the seat 502 of the downhole tool 500. As soon as the drop ball 402 sealingly engages with the seat 502, the hydraulic pressure of the fluid pushes the flange member 410 towards the drop ball 402 simultaneously actuating the centralizing member 406 so as to bend and engage the spreading elements 408 with the interior wall of the throughbore. The increasing hydraulic pressure opens the flow port 504 so that fracing fluid flows into the wellbore. The fracing fluid props open any fractures within the isolated region of the wellbore so that hydrocarbons can flow freely into the wellbore. Temperature and pressure changes during the fracing operation may initiate data acquisition of the data acquisition device 404, which also starts to record the data. Alternatively, the data acquisition device 404 may be operated from the surface remotely, or may be on a timer to operate after a period of time has elapsed.

When the fracing operation is completed and hydrocarbons flow into the wellbore, the production fluid is pumped uphole (see FIG. 22). The fluid flowing uphole engages with the uphole surface 412 of the flange member 410 which slides in a direction away from the drop ball 402 thus disengaging the spreading elements 408 from the interior wall of the throughbore so that the drop device 400 is pumped uphole with the fluid flow. Once recovered, the data stored on the drop device 400 can be downloaded for assessment before reusing the drop device 400. Alternatively, the data acquisition device 404 and centralizing member 406 may be coupled to a different sized drop ball 402 for engagement with a different downhole tool.

In an alternative method for recovering the drop device 400, a milling device (e.g. mill plug) 600, similar in design to the milling device 300 described with the third aspect of the invention and shown in FIG. 13, may be used in the same way as described for the third aspect of the invention. In particular, as shown in FIG. 23, once the fracing operation has been completed, the milling device tool 600 is sent downhole, for example, on coiled tubing or a drill pipe string to recover the data acquisition device 404 and pick-up each downhole tool 500 in turn starting with the uppermost one closest to the surface.

The procedure described for the drop device 400 may be repeated for each of the isolated regions covered by the multi stage fracturing system.

It will be appreciated by persons skilled in the art that the above embodiments have been described by way of example only and not in any limitative sense, and that various alterations and modifications are possible without departing from the scope of the invention. 

1. A downhole device for data acquisition during fracing operations, the device comprising: a bypass device operatively coupleable to a wellbore selective obturation member and adapted to provide a selectively closable first fluid path for fluid communication through said wellbore selective obturation member between a region inside the throughbore of the wellbore located uphole of said downhole device and a region inside the wellbore located downhole of said downhole device when in situ; and at least one data acquisition device, adapted to detect at least one physical property and operatively coupled to said bypass device, wherein said bypass device is further adapted to provide at least one second fluid path for fluid communication between said at least one data acquisition device and said region inside the wellbore located uphole of said bypass device.
 2. The downhole device according to claim 1, wherein said at least one data acquisition device is operatively coupled to said bypass device directly or via a data acquisition device carrier mountable to said bypass device and adapted to accommodate said at least one data acquisition device.
 3. The downhole device according to claim 1, further comprising a centralizing member operatively coupled to said at least one data acquisition device and adapted to centralize and/or rotationally stabilize said at least one data acquisition device within the wellbore.
 4. The downhole device according to claim 2, further comprising a centralizing member operatively coupled to said data acquisition device carrier and adapted to centralize and/or rotationally stabilize said data acquisition device carrier within said wellbore.
 5. The downhole device according to claim 1, wherein a seat member located within or at an end of said first fluid path is adapted to engage with a drop member so as to at least unidirectionally selectively prevent fluid flow through said first fluid path.
 6. The downhole device adapted according to claim 1, wherein said second fluid path is adapted to bypass said first fluid path.
 7. The downhole device according to claim 1, wherein said at least one data acquisition device comprises an integrated data storage device adapted to record a predetermined maximum amount of data.
 8. The downhole device according to claim 7, wherein said at least one data acquisition device comprises a data interface adapted to permit download of at least part of said predetermined maximum amount of data to a remote data storage means.
 9. The downhole device according to claim 1, wherein said at least one data acquisition device is adapted to transmit data to a remote location uphole of said downhole device within the wellbore.
 10. The downhole device according to claim 1, wherein said at least one data acquisition device is any one or all of a pressure data acquisition device and/or a temperature data acquisition device and/or a geophone and/or a micro seismic sensor.
 11. The downhole device according to claim 2, wherein said data acquisition device carrier is adapted to irretractably engage with a data acquisition device catcher mechanism when moving along a longitudinal axis inside a pick-up device of said data acquisition device catcher mechanism.
 12. The downhole device according to claim 11, wherein said data acquisition device carrier comprises at least one recess or shoulder or other formation provided on or in the outer surface of said data acquisition device carrier and is adapted to cooperate with said data acquisition device catcher mechanism.
 13. The downhole device according to claim 12, wherein said data acquisition device catcher mechanism comprises at least one spring loaded catcher member.
 14. The downhole device according to claim 1, wherein said bypass device is located inside a mandrel of said wellbore selective obturation member when in situ.
 15. The downhole device according to claim 14, wherein at least one of said data acquisition device and the data acquisition device carrier is located downhole of said bypass device.
 16. The downhole device according to claim 1, wherein said bypass device is located uphole of said wellbore selective obturation member when in situ.
 17. The downhole device according to claim 16, wherein said at least one data acquisition device and the data acquisition device carrier is operatively located inside a mandrel of said wellbore selective obturation member.
 18. The downhole device according to claim 1, wherein said bypass device is located downhole of said wellbore selective opturation member when in situ.
 19. The downhole device according to claim 18, wherein said at least one data acquisition device and the data acquisition device carrier is located downhole of said bypass device.
 20. The downhole device according to claim 2, wherein said second fluid path comprises an incompressible fluid sealingly encased within said second fluid path and said data acquisition device carrier by a membrane located at the uphole end of said second fluid path.
 21. The downhole device according to claim 20, wherein said membrane is typically pressure sensitive such that it can transmit pressure from one side to the other side of said membrane.
 22. The downhole device according to claim 20, wherein said membrane is sufficiently flexible and diathermic to prevent distortion of said physical properties that are measured with said at least one data acquisition device.
 23. A method for acquiring data during fracing operations in a subterranean formation having a wellbore penetrating the formation, the method comprising: (a) running in a downhole device having a data acquisition device into a wellbore along with a wellbore selective obturation member; (b) placing said downhole device and wellbore selective obturation member at a location inside the wellbore; (c) initiating at least one data acquisition device of said downhole device for monitoring at least one physical property; (d) initiating a fracing operation while monitoring said at least one physical property; and (e) recovering said monitored physical property after the fracing operation is completed.
 24. The method according to claim 23 wherein the downhole device comprises: a bypass device operatively coupleable to a wellbore selective obturation member and adapted to provide a selectively closable first fluid path for fluid communication through said wellbore selective obturation member between a region inside the throughbore of the wellbore located uphole of said downhole device and a region inside the wellbore located downhole of said downhole device when in situ; and at least one data acquisition device, adapted to detect at least one physical property and operatively coupled to said bypass device, wherein said bypass device is further adapted to provide at least one second fluid path for fluid communication between said at least one data acquisition device and said region inside the wellbore located uphole of said bypass device.
 25. The method according to claim 24, wherein there is an additional step in between steps c) and d) of:— closing a first fluid path for fluid communication through the wellbore selective obturation member between a region inside the wellbore located uphole of said downhole device and a region inside the wellbore located downhole of said downhole device when in situ; and wherein step (d) includes monitoring said at least one physical property through said at least one second fluid path.
 26. The method according to claim 23, wherein said at least one data acquisition device is adapted to record a predetermined amount of data of said at least one physical parameter.
 27. The method according to claim 23, wherein said at least one data acquisition device is adapted to transmit said predetermined amount of data to a remote data storage means.
 28. The method according to claim 23, wherein said at least one monitored physical property is recovered via a remotely located communication device operatively connectable to said at least one data acquisition device.
 29. A method for recovering at least one downhole device having a data acquisition device from a downhole location inside a wellbore typically after completing fracing operations, the method comprising: (a) running a plug milling device, comprising at least one data acquisition device retrieval mechanism, downhole inside a wellbore; (b) engaging a first of said at least one downhole device so as to irretrievably move a data acquisition device carrier and/or data acquisition device of said at least one downhole device into and along a longitudinal axis of said data acquisition device retrieval mechanism, milling away any parts of said at least one downhole device exceeding an inner diameter of said data acquisition device retrieval mechanism; and (c) removing said plug mill including said at least one downhole device from said wellbore.
 30. A method according to claim 29, wherein the downhole device comprises: a bypass device operatively coupleable to a wellbore selective obturation member and adapted to provide a selectively closable first fluid path for fluid communication through said wellbore selective obturation member between a region inside the throughbore of the wellbore located uphole of said downhole device and a region inside the wellbore located downhole of said downhole device when in situ; and at least one data acquisition device, adapted to detect at least one physical property and operatively coupled to said bypass device, wherein said bypass device is further adapted to provide at least one second fluid path for fluid communication between said at least one data acquisition device and said region inside the wellbore located uphole of said bypass device.
 31. The method according to claim 29, further comprising a step between steps (b) and (c) of engaging any subsequent second of said at least one downhole device so as to irretrievably move at least one of a data acquisition device carrier and a data acquisition device of said second of said at least one downhole device into and along a longitudinal axis of said data acquisition device retrieval mechanism, milling away any parts of said second of said at least one downhole device exceeding an inner diameter of said data acquisition device retrieval mechanism.
 32. The method according to claim 29, wherein said plug milling device and data acquisition device retrieval mechanism is adapted to be operated on coiled tubing or drill pipe or any other suitable work string.
 33. The method according to claim 29, wherein said data acquisition device retrieval mechanism comprises at least one catcher mechanism adapted to engage with said at least one of the at least one data acquisition device and the data acquisition device carrier of said downhole device so as to prevent downhole movement of said data acquisition device and/or data acquisition device carrier with respect to said data acquisition device retrieval mechanism in situ.
 34. The method according to claim 33, wherein said catcher mechanism is at least one spring loaded catcher member adapted to engage with at least one recess or shoulder located on the outer surface of said at least one data acquisition device and the data acquisition device carrier.
 35. A drop device for data acquisition during fracing operations, the drop device comprising: an obturation member adapted to at least one of actuatingly and sealingly engage with at least one downhole tool; and at least one data acquisition device operatively coupled to said obturation member and adapted to detect at least one physical property.
 36. The drop device according to claim 35, wherein said obturation member is adapted to sealingly engage with a seat member of said downhole tool so as to prevent fluid communication through said downhole tool between a region inside the throughbore of the wellbore located uphole of said obturation member and a region inside the wellbore located downhole of said obturation member when in situ.
 37. The drop device according to claim 35, wherein said at least one data acquisition device is located uphole of said obturation member when in situ.
 38. The drop device according to claim 35, wherein said at least one data acquisition device is operatively coupled to said obturation member via a data acquisition device carrier removably mountable to said obturation member and adapted to accommodate said at least one data acquisition device.
 39. The drop device according to claim 35, further comprising a centralizing member operatively coupled to said at least one data acquisition device and adapted to at least one of centralize, fixate, and rotationally stabilize said at least one data acquisition device within the wellbore.
 40. The drop device according to claim 39, wherein said centralizing member is operatively coupled to said at least one data acquisition device via said data acquisition device carrier.
 41. The drop device according to claim 39, wherein said centralizer member comprises a plurality of spreading elements adapted to spread radially towards and in engagement with the inner wall of at least one of the wellbore and the drill string.
 42. The drop device according to claim 35, further comprising a flange member operatively coupled to said at least one data acquisition device and adapted to provide a contact surface for fluid flowing through the wellbore.
 43. The drop device according to claim 42, wherein said contact surface is selectively retractable so as to allow fluid flow between a region located uphole of said contact surface and a region located downhole of said contact surface.
 44. The drop device according to claim 43, wherein said contact surface comprises an uphole contact surface adapted to operatively engage with fluid flowing in an uphole direction, and a downhole surface adapted to operatively engage with fluid flowing in a downhole direction when in situ.
 45. The drop device according to claim 42, wherein said flange member is adapted to actuate said centralizing member.
 46. The drop device according to claim 42, wherein said flange member is adapted to slidingly actuate said centralizing member.
 47. The drop device according to claim 35, wherein said at least one data acquisition device comprises an integrated data storage device adapted to record a predetermined maximum amount of data.
 48. The drop device according to claim 47, wherein the at least one data acquisition device comprises a data interface adapted to permit download of at least part of said predetermined maximum amount of data to a remote data storage means.
 49. The drop device according to claim 35, wherein said data acquisition device is further adapted to transmit data to a remote location uphole of said downhole tool within the wellbore.
 50. The drop device according to claim 35, wherein said at least one data acquisition device is any one or all of a pressure data acquisition device, a temperature data acquisition device, a geophone, and a micro seismic sensor.
 51. The drop device according to claim 42, wherein said flange member is a wiper.
 52. A method for acquiring data during fracing operations in a subterranean formation having a wellbore penetrating the formation, the method comprising: (a) providing the drop device according to claim 35 at a predetermined location and in engagement with a predetermined downhole tool; (b) initiating at least one data acquisition device of said downhole device for monitoring at least one physical property; (c) closing a first fluid path for fluid communication through the downhole tool between a region inside the wellbore located uphole of said drop device and a region inside the wellbore located downhole of said drop device when in situ; (d) initiating a fracing operation while monitoring said at least one physical property through at least one data acquisition device of said drop device; and (e) recovering said monitored physical property after the fracing operation is completed.
 53. The method according to claim 52, wherein said drop device is recovered by fluid flowing uphole through at least one of the wellbore and the drill string and engaging with a contact surface of a flange member of said drop device.
 54. The method according to claim 52, wherein said drop device is recovered by running a milling device comprising at least one data acquisition device retrieval mechanism downhole inside a wellbore.
 55. A frac plug comprising a downhole device according to claim
 1. 